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Future abrupt reductions in the summer Arctic sea ice
Marika M. Holland
National Center for Atmospheric Research,
Boulder, Colorado, USA
Cecilia M. Bitz
Atmospheric Sciences, University of Washington,
Seattle, Washington, USA
Bruno Tremblay
Lamont Doherty Earth Observatory of Columbia University,
Palisades, New York, USA
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<2> Arctic sea ice has undergone dramatic changes in recent years with considerable thinning of the ice pack , a sharp reduction in the multi-year ice area , and record minimum September ice cover . These changes have led to the suggestion that a “tipping point” may have been reached in which strong positive feedbacks accelerate ice retreat and result in an era of thinner, less extensive ice cover in the Arctic . However, the patchy observational record and considerable natural variability in the Arctic make it difficult to assess whether a tipping point has actually been reached.
<3> Evidence is mounting that the observed changes are associated with anthropogenically driven climate change and climate models predict Arctic change to continue into the foreseeable future . The transition from perennial to seasonal Arctic ice cover has numerous implications for the climate system. Additionally, the rate and manner in which sea ice retreats affects the ability of ecosystems and societies to adapt to these changes. Here we examine the potential for abrupt transitions in the future Arctic summer sea ice from climate models that have contributed output to the Intergovernmental Panel on Climate Change fourth assessment report (IPCC-AR4).
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<7> The CCSM3 simulations compare well to the observed ice cover including the rate of its recent retreat (Figure 1a) . The simulations do not however indicate that ice retreat will continue at a constant rate into the future. Instead, they show abrupt transitions that suggest near ice-free Septembers could be reached within 30–50 years. The simulated changes are surprisingly rapid. To illustrate these changes and the mechanisms driving them, we present the results from one realization (Run 1) of a group of seven ensemble members. To demonstrate the robustness of the results, we evaluate other ensemble members of the same model and simulations from other models.
<8> In the 20th century, the rate of the simulated September ice retreat is in accord with observations (Figure 1a). From 1979–2005, the Run 1 ice extent decreases by 10% per decade, which is consistent with the observed 8% per decade decrease when accounting for intrinsic variability as assessed from the different ensemble members. The late-20th century Arctic is mostly covered with perennial ice, with reduced concentration in summer along the shelves where first year ice melts away (Figure 1b). The simulated ice declines rapidly from 1998 to 2003, losing 20% of its extent in 6 years. The rate of change then becomes more modest again until 2024. From 2003–2024, the simulated Arctic (Figure 1b) still has more than 60% perennial coverage, although, compared to the late 20th century, the September ice concentration is reduced with large open water areas along the Arctic shelves. Starting in 2024, the September ice retreats rapidly from approximately 6 million km2 to 2 million km2 in a decade (Figure 1). Over this event, the trend of the 5-year running mean smoothed timeseries is −0.4 million km2 per year, which is over 3 times larger than any comparable trend in any 10-year interval of the observed 1979–2005 record and about 5 times larger than any comparable 10-year trend of the simulated 20th century timeseries. After this event, by 2040, a small amount of perennial ice remains along the north coast of Greenland and Canada, leaving the majority of the Arctic basin ice free in September (Figure 1b).
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